A major problem exists in the use of single and multiple trailers being towed one behind the other in truck-trailer or road train configurations. These vehicles are commonly referred to as combination vehicles, typically comprising either a rigid truck or prime mover and one or more semi-trailers and converter dollies.
In this specification the following explanations apply to certain terms:                a) A “semi-trailer” is a towed vehicle unit of a heavy goods combination vehicle whose means of attachment to the preceding vehicle unit (a prime mover, another semi-trailer, or a converter dolly) results in some of its vertical load being imposed on said preceding unit through a tow coupling and whose rear is supported by a single axle or multiple axle bogie (tandem, tri, quad, etc.) located towards the rear.        b) A “converter dolly” (also called a “dolly”) is a towed vehicle unit of a heavy goods combination vehicle whose tow coupling to the preceding vehicle unit (a rigid truck, a prime mover or another semi-trailer) results in little or none of its vertical load being imposed on said preceding vehicle unit through the tow coupling and whose rear is supported by a single axle or multiple axle bogie (tandem, tri, quad, etc.) located at or near to the centre of the main load carrying area. The main load bearing or carrying area of a converter dolly usually incorporates a tow coupling device so that the dolly may be mounted beneath the front of a semi-trailer body, thereby converting the semi-trailer into a trailer with two axle groups of which the front axle group is steered by connection to the preceding vehicle unit. Internationally, a converter dolly is also known as a pivoting bogie, a dolly truck or a special truck. A converter dolly may also be adapted to carry goods.        c) A “trailer” in a heavy goods combination vehicle is a towed vehicle unit that can be either a semi-trailer, or a combination of a converter dolly plus a semi-trailer, or a converter dolly adapted to carry goods.        d) A “motor vehicle” in a heavy goods combination vehicle is a vehicle unit which is motorised for propulsion and is built to tow one or more trailers. One of the trailers may be a powered trailer. A motor vehicle can be a prime mover or a rigid truck.        e) A “powered trailer” in a heavy goods combination vehicle is a trailer with a largely self-contained means of generating propulsion through one or more of its wheels. In a combination vehicle one or more of the trailers may be a powered trailer.        f) A “prime mover” is a vehicle unit of a heavy goods combination vehicle built to tow a semi-trailer or a converter dolly. A prime mover may also carry a load.        g) A “rigid truck” is a non-articulated motor vehicle built to carry a load and, in a heavy goods combination vehicle, tow a trailer.        h) A “vehicle unit” is either a trailer, a semi-trailer, a converter dolly, a motor vehicle, or a powered trailer. Thus a combination vehicle has a plurality of such vehicle units connected by an articulated coupling between each adjacent vehicle unit.        i) A “heavy goods combination vehicle” is a heavy goods or road transport vehicle with at least one articulation point.        
A dynamic characteristic unique to combination vehicles is a tendency in a steering manoeuvre for each trailer in the combination to experience a higher level of lateral acceleration than the unit preceding it. This is referred to as rearward amplification and is a serious safety issue particularly in rapid path-change manoeuvres or abrupt steering events as it can lead to rear-trailer rollover. Rearward amplification improves with fewer articulation points and is strongly influenced by a range of mechanical properties, including trailer and dolly wheelbase dimensions, the type of tow coupling between vehicle units and the location of tow coupling points, tyre cornering stiffness, suspension mechanical properties, centre-of-gravity height and all-up weight.
Current designs and attempts to increase loads in trailers have led to rollover incidents in which a second or subsequent trailer overreacts to a steering deviation by the driver through the rearward amplification mechanism and rolls over. Particularly prone to this is the rearmost trailer, and to a lesser extent the rear trailer set if two or more semi-trailers are connected together in roll through a turntable (in which case the relevant vehicle units are said to be roll coupled).
An aim of the present invention is to provide a means of steering the wheels on the dollies of such trailers, not being the first vehicle unit in the combination vehicle, which reduces the likelihood and danger of such rollovers and allows safe transport of current and greater loads in trailers.